The present invention relates to a slippage control system for a lock up clutch assembly for a fluid coupling such as a torque converter for an automatic transmission for an automotive vehicle such as an automobile, and more particularly relates to such a slippage control system for such a fluid coupling lock up clutch assembly, particularly which allows for a certain amount of slippage of said lock up clutch assembly even when said lock up clutch assembly is engaged, in order to minimize transmission torque shock and vibration.
In the prior art, there have been proposed various types of lock up clutch assembly for a fluid coupling such as a torque converter for an automatic transmission for a vehicle. In Japanese Patent Laying Open Publication Serial No. 57-40162 (1982), which it is not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, there is disclosed a lock up clutch assembly for a torque converter for an automatic transmission for a vehicle, said lock up clutch assembly being selectively operated so as either to be substantially fully disengaged so as not to provide any substantial dragging, or so as to be substantially fully engaged so as not to provide any substantial slippage. In such a system, when said lock up clutch assembly is substantially fully engaged, the rotational drive and torque supply from the engine of the vehicle are transmitted straight through said lock up clutch assembly from the input of the torque converter to the output of said torque converter, without said lock up clutch assembly providing any substantial slippage between said torque converter input and output, and without any torque conversion or rotational speed alteration or torque shock cushioning function of the torque converter being provided, and thereby the drive losses in said torque converter are reduced to a minimum, and drive train efficiency and also vehicle fuel consumption and responsiveness are maximized.
However, in such a system in which the lock up clutch assembly of the torque converter is provided so as, when engaged, to directly and positively couple together the input member of the torque converter and the output member thereof without allowing any substantial slippage at all therebetween, the problem arises that in such an operational mode the rotational vibration and torque fluctuations which are inevitably present in the power output of the internal combustion engine of the vehicle upon the input side of the torque converter are directly transmitted to the output side of said torque converter and to the automatic transmission and the differential device of the vehicle, which is not desirable since it causes the vibration of the vehicle body as sensed within the passenger compartment thereof to be increased, as well as increasing the noise level within the vehicle and deteriorating the ridability thereof. Further, the stress upon the various components of the automatic transmission and of the differential device of the vehicle is increased, which can deteriorate the reliability and the service life thereof.
Accordingly there was proposed, for instance in Japanese Patent Laying Open Publication Serial No. 57-157860 (1982), which also it is not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, a slippage control system for a fluid coupling lock up clutch assembly, which was of the electronic type, and according to the operation of which a slight relative rotation was permitted between the input member of the torque converter and the output member thereof, even when the lock up clutch assembly of said torque converter was in the so called locked up state. With such a slippage control system for a fluid coupling lock up clutch assembly, when the lock up clutch assembly is in the so called locked up condition, then nevertheless a certain amount of relative rotation is permitted between the input member of the torque converter and the output member thereof, thus cushioning between said input member and said output member of said torque converter and inhibiting or substantially preventing rotational vibration and torque fluctuations present in the power output of the internal combustion engine of the vehicle from being directly transmitted through said lock up clutch assembly to the automatic transmission and thence to the differential device of the vehicle. Thereby, vibration of the vehicle body as sensed within the passenger compartment is kept low, and also the noise level within the vehicle passenger compartment is minimized, whereby the ridability of the vehicle is enhanced. Further, the stress upon the various components of the automatic transmission and of the differential device of the vehicle is reduced, thus enhancing the reliability and the service life thereof.
There is however a problem with such a slippage control system for a fluid coupling lock up clutch assembly, in that sensors are required to be provided for detecting the rotational speeds of the input and the output members of the torque converter, and the outputs from these rotational speed sensors are then fed to a microcomputer which is also required to be provided, which computes the difference between said rotational speeds of said input and output members of said torque converter, and which then controls the hydraulic fluid pressure which is supplied as an actuating hydraulic fluid pressure to the lock up clutch assembly according to this rotational speed difference, so as (for example) to maintain said rotational speed difference so as to be within a determinate range, i.e. so as to be neither too high nor too low. This form of construction inevitably causes the slippage control system for the lock up clutch assembly for the torque converter to be relatively complicated, accordingly raising its inherent cost and also increasing the cost of assembly of said slippage control system to the vehicle. Further, since various electrical components such as rotational speed sensors and a microcomputer and the like are required to be incorporated in such a slippage control system of an electronic type construction, its reliability is inevitably not as high as could be desired.